The present invention relates generally to semiconductor lasers, and, in particular, to methods and circuits to decrease the turn off time for a vertical cavity surface emitting laser.
Semiconductor lasers are widely used in high speed data communications. Modulated light from the lasers are used to carry information through fiber optic lines. For some data formats, generally, when a laser emits light the data value is considered a logical one and when the laser is largely off the data value is considered a zero.
Vertical cavity surface emitting lasers (VCSELs) are one type of laser used in data communication networks. VCSELs are generally relatively easy to manufacture using semiconductor processes. Drive circuitry for VCSELs provide a VCSEL with sufficient current to turn xe2x80x9conxe2x80x9d, i.e., causing the VCSEL to emit light. Likewise, the drive circuitry removes or prevents current from flowing to the VCSEL to turn the VCSEL to turn xe2x80x9coffxe2x80x9d, i.e., causing the VCSEL to largely not emit light. However, when VCSELs turn on, electrical charge is stored on the anode of the VCSEL. Removing this electrical charge decreases the turn-off time of the VCSEL, and thereby increases the maximum data rate the VCSEL can support. Furthermore, removing the excess charge can be difficult as it is often desirable to maintain a low bias current when the VCSEL is in the xe2x80x9coffxe2x80x9d state. The bias current allows the VCSEL to be turned on faster. Thus, although the extra electrical charge is removed from the VCSEL to turn off the VCSEL, bias current to the VCSEL still should be maintained.
The present invention provides methods and systems for driving semiconductor lasers such that turn-off time of a laser is decreased. In one embodiment, a drive circuitry that drives a semiconductor laser is provided. The drive circuitry includes a modulator coupled to the semiconductor laser and generates an output signal to control the semiconductor laser. A negative peak timer is coupled to the modulator and a limiter is coupled to the negative peak timer and the modulator. The negative peak timer causes the modulator to rapidly decrease magnitude of the output signal of the modulator to turn off the semiconductor laser.
In another embodiment, a drive circuitry is provided that drives a semiconductor laser. The drive circuitry includes a limiter which receives a differential input and is configured to generate first differential output signals and second differential output signals. A negative peak timer is coupled to the limiter and receives the first differential signals from the limiter. The negative peak timer is also configured to generate third differential output signals. A modulator is also coupled to the limiter and the negative peak timer and receives the second differential output signals from the limiter and the third differential output signals from the negative peak timer. The modulator is also configured to generate an output pulse. A vertical cavity surface emitting laser is coupled to the modulator and receives the output pulse from the modulator to turn the laser on and off. The modulator is also configured to remove excess charge stored when the vertical cavity surface emitting laser is turned off. In one aspect of the invention, the output pulse is a voltage pulse that has an adjustable undershoot. The adjustable undershoot is determined by a negative peaking pulse from the negative peak timer.
In another embodiment, a method of driving a semiconductor laser is provided. An output signal is generated from a modulator to control the semiconductor laser. The modulator causes a rapid decrease in magnitude of the output signal of the modulator to turn off the semiconductor laser.
Many of the attendant features of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in conjunction with the accompanying drawings.